Active noise cancellation (ANC) is a conventional method of reducing an amount of undesired noise received by a user listening to audio through headphones. The noise reduction is typically achieved by playing an anti-noise signal through the headphone's speakers. The anti-noise signal is an approximation of the negative of the undesired noise signal that would be in the ear cavity in the absence of ANC. The undesired noise signal is then neutralized when combined with the anti-noise signal.
In a general noise-cancellation process, one or more microphones monitor ambient noise or residual noise in the ear cups of headphones in real-time, then the speaker plays the anti-noise signal generated from the ambient or residual noise. The anti-noise signal may be generated differently depending on factors such as physical shape and size of the headphone, frequency response of the speaker and microphone transducers, latency of the speaker transducer at various frequencies, sensitivity of the microphones, and placement of the speaker and microphone transducers, for example.
In feedforward ANC, the microphone senses ambient noise but does not appreciably sense audio played by the speaker. In other words, the feedforward microphone does not monitor the signal directly from the speaker. In feedback ANC, the microphone is placed in a position to sense the total audio signal present in the ear cavity. So, the microphone senses the sum of both the ambient noise as well as the audio played back by the speaker. A combined feedforward and feedback ANC system uses both feedforward and feedback microphones.
For optimal noise rejection performance, the filter gain values of the feedforward and the feedback ANC paths generally are precisely tuned. Even so, the gain in an ANC path may differ from one part to another. These differences may be due to variations in the sensitivity or efficiency of the speaker and microphone transducers. If the feedforward ANC gain is too high, ambient noise may bleed in to the headphone. Also, if the feedback ANC gain is too high, there may be an increased hiss noise or loud spontaneous oscillations in the audio played by the speaker. On the other hand, if the feedback ANC gain or the feedforward ANC gain is too low, there may be a reduced amount of noise cancellation.
Even after calibration, the feedback ANC gain may increase or decrease from the tuned value. If the gain increases, the feedback ANC path may spontaneously oscillate, with the amplitude of the oscillation limited only by the full scale.
Embodiments of the invention address these and other issues in the prior art.